MITF is a master transcriptional regulator of melanocyte pigmentation as well as lineage survival. Prior studies supported by this grant have demonstrated that loss-of-function mutations produce accelerated hair- graying or complete absence of melanocytes, whereas MITF gene amplification is an oncogenic event in a subset of human melanomas. We have also recently discovered MITF to be translocated in a human non- melanoma malignancy which expresses melanocytic markers, and identification of the fusion partner is underway. Although MITF regulates either pigmentation or survival, its transcriptional targets and mechanisms responsible for these alternative activities remain largely unknown. MITF does directly regulate expression of multiple pigmentation-related genes, leading to the discovery that MITF expression is regulated by the Melanocyte Stimulating Hormone (MSH) pathway. The MSH receptor (MC1R) is a G Protein Coupled Receptor which, if mutated to prevent cAMP induction, produces common red/blond pheomelanin pigmentation and fair-skinned phenotype in rodents and man. We have generated a novel mouse "fairskinned" model which incorporates the K14-SCF transgene (producing "humanized" skin with epidermal melanocytes). When combined with deficiency of the XPC DMA repair gene, we observed UV- induced melanoma formation at an incidence which was significantly higher in fairskinned (pheomelanotic) mice than in unpigmented albinos. This observation matches human epidemiologic evidence that pheomelanin may participate in melanoma formation. Moreover we observed that UV-induced pigmentation (tanning) is absolutely dependent upon MSH receptor function, and dark (eumelanin) pigmentation could be efficiently rescued by topical administration of a cAMP agonist to fairskinned (pheomelanotic) mice. This "sunless tanning" profoundly protected genetically pheomelanotic mice against UV skin carcinogenesis. Based upon these studies we propose the following aims: 1) Extend the analysis of pheomelanin's role in UV skin carcinogenesis to the XPC wildtype genetic background (which is more relevant to most human melanoma), 2) Utilize a systematic genomics approach to identify all G protein coupled receptors for which agonists may rescue cAMP and dark pigmentation in me 1r fairskinned variants, 3) Examine the mechanism underlying UV induction of MSH in keratinocytes, 4) Employ whole genome chromatin immunoprecipitation to identify MITF occupancy and target genes under different conditions of melanocyte growth, differentiation, or transformation, and 5) Identify MITF's fusion partner in primary tumors containing FISH-proven gene translocations.